Development Of ZIF-8 Slurry With High Selectivity And Sorption Speed And Its Application In Gas Separation

In recent years,climate and environmental problems caused by the global greenhouse effect have become increasingly serious.Under the current"carbon peak and neutrality targets"policy,China’s traditional energy industries such as fossil fuel power generation and petroleum refining are facing enormous pressure.On the one hand,fossil fuel power plants emit 40%of the world’s CO₂,which is the main culprit for the aggravation of the greenhouse effect.On the other hand,the high value-added light hydrocarbons by-produced in the petroleum refining,such as mixed C4 hydrocarbons,are mostly not used for further chemical utilization due to the separation difficulty,but are directly burned as fuel,resulting in a serious waste of resource,and emit a large amount of CO₂,further aggravating the greenhouse effect.At present,the application bottleneck of CO₂ capture and C4 hydrocarbons separation technology is the high cost of enterprises caused by high energy consumption.In view of this situation,this paper develops a novel CO₂ capture and C4 hydrocarbons separation technology with high selectivity and sorption speed but low energy cost by suspending the solid phase adsorbent in the suitable liquid absorbent for gas separation.The main work of this paper is as follows:（1）A new ZIF-8/N,N-Dimethylacetamide（DMAC）slurry was developed by solvent screening.The performance of the ZIF-8 slurry for capturing CO₂ and separating integrated gasification combined cycle（IGCC）syngas（H₂/CO₂）were investigated.The results show that the preferred ZIF-8（40 wt%）/DMAC slurry exist low viscosity（<13MPa·s）and high CO₂ solubility.For example,it reaches 4.56 mol/L at 298.15 K and 1.5MPa,while the desorption heat is only 16 k J/mol,indicating that the dissolving of CO₂in the slurry is a pure physical process.The CO₂ solubility of the slurry can be doubled than that of pure DMAC or traditional physical absorbent propylene carbonate（PC）,and even exceed that of monoethanolamine（MEA）aqueous solution（20 wt%）in medium and high pressure.More importantly,the slurry exhibits higher CO₂ sorption speed than PC and MEA aqueous solutions.The equilibrium separation results on H₂/CO₂ mixtures using ZIF-8/DMAC slurry show that the selectivity of CO₂ over H₂ ranged from 22 to119;it increases with decreasing temperature or pressure,increasing initial gas to slurry ratio or CO₂ concentration in feed gas.（2）Another kind of ZIF-8/M-based slurry with high stability was developed and applied to the separation of butane isomers,where M is an organic solvent with excellent properties.The pure butane sorption and two-component butane gas mixtures separation on ZIF-8 powder,ZIF-8/water slurry,ZIF-8/M slurry,and ZIF-8/M-water slurry were compared.The results show that ZIF-8 powder possesses the highest separation selectivity of 11757,while ZIF-8/M slurry exist the fastest kinetic speed,but the separation factor is only 113.But it is larger than those of all advanced membrane materials reported in the literature.On the basis of ensuring excellent kinetic performance,the further improved ZIF-8/M-water slurries greatly improve the separation ability of n-/iso-butane gas mixtures.The separation factors can reach more than 890.Through single-stage separation,the iso-butane concentration can be increased from 61.8 mol%of the feed gas to 98.65 mol%,and the n-butane recovery rate can reach more than 97.9%.The separation performance of the ZIF-8/M-water slurry did not change at all in 22 sorption-desorption cycle experiments in 14 days.（3）The sorption kinetics of ZIF-8/M-water slurry for C4 hydrocarbons were studied,and the mechanism of the sorption speed of the slurry being superior to that of ZIF-8powder was expounded.The effects of particle size,particle agglomeration and particle shaping of ZIF-8 powder on its adsorption kinetics were investigated.The results show that the solvent M has a good dispersing effect on the agglomerated ZIF-8,thereby improving its adsorption speed.Large size,particle agglomeration and shaping are not conducive to the mass transfer of ZIF-8.In addition,the gas-solid-liquid three-phase mass transfer process of the slurry system and the mass transfer enhancement effect of the solid-phase adsorbent are discussed.（4）The breakthrough experiments of n/iso-butane gas mixture and C4 olefin multi-component gas mixtures in ZIF-8/M-water slurry were carried out,and the effects of temperature,pressure and gas flow rate on the breakthrough results were investigated.The results show that the breakthrough results of n/iso-butane mixture in ZIF-8 slurry are very good,verifing the excellent kinetic selectivity of the slurry,and the lower the temperature and pressure,the better the breakthrough effect.However,the breakthrough results for C4 olefins are not as significant as those for n/iso-butane.Except for iso-butene,other components are not easily separated.（5）The n/iso-butane separation process by ZIF-8/M-water slurry was modeled.The slurry separation is divided into two processes:absorption and adsorption.The adsorption part adopts the Langmuir adsorption model,and the absorption part adopts the three-parameter Patel-Teja（P-T）equation of state combined with the van der Waals single-fluid mixing rule to calculate the gas/liquid phase fugacity.After fitting by experimental data,the three parameters of the Langmuir equation for the n-butane adsorption by ZIF-8 and the binary interaction parameters between M,water and n-butane（iso-butane）in P-T EOS were obtained.The ZIF-8 powder and ZIF-8/M-water slurry separation results for n/iso-butane were predicted using the obtained thermodynamic model.The predicted values are in good agreement with the experimental values,and the average relative error is4.19%,which can meet the requirements of industrial precision.（6）A set of pilot scale continuous gas separation experimental device was designed and built,and the pilot test of C4 hydrocarbon mixture separation using slurry method was carried out.To the best of our knowledge,it should be the first pilot industrial test using metal organic frameworks（MOFs）for gas mixture separation in the world.The main body of the separation device is two packed columns with an overall height of 5.5m,of which the inner diameters of the absorption column and desorption column are 47mm and 98 mm respectively.The maximum working pressure of the device is 5 MPa and the working temperature is 253.15 k-373.15 K.The separation effect of the pilot plant was preliminarily investigated.Under an experimental condition,the product gas（iso-butane）concentration can be concentrated from 51.19 mol%of the feed gas to 99.46mol%,and the separation factor of iso-butane over other components can reach 1329.The calculated iso-butane yield can reach 87%.As a comparison,the purity and yield of is-obutane product by distillation method with similar feed gas composition is only 98.54mol%and 54%.In addition,during the 7-month investigation period,the slurry ran well,and there was no packing blockage and separation performance degradation,which further verified the feasibility of industrial application of the slurry method.